Refrigerant system with variable speed compressor in tandem compressor application

ABSTRACT

A refrigerant system is provided with tandem compressors. As is known, tandem compressors operate in parallel to provide a refrigerant system designer with the ability to achieve a stepped capacity control of the refrigerant system. At least one of the tandem compressors is provided with a variable speed drive. Further, at least one of the tandem compressors may be provided with the economizer and/or unloader functions. System configurations with multiple compression stages and multiple injection ports are disclosed. In this manner, the stepless capacity control can be achieved.

BACKGROUND OF THE INVENTION

This invention relates to a variable speed motor for driving acompressor that is incorporated into a refrigerant system with tandemcompressors.

Refrigerant systems are utilized in many air conditioning and heat pumpapplications for cooling and/or heating the air entering an environment.The cooling or heating load on the environment may vary with ambientconditions, and as the temperature and/or humidity levels demanded by anoccupant of the building change.

In some refrigerant systems, a single compressor is utilized to compressthe refrigerant and move the refrigerant through the cycle connectingindoor and outdoor heat exchangers in a closed loop. However, under manycircumstances, it would be desirable to have the ability to vary thecapacity, or amount of cooling or heating provided by the refrigerantsystem. Thus, known refrigerant systems may be provided with tandemcompressors. Tandem compressors are essentially at least two compressorsoperating in parallel, where the compressors are interconnected witheach other via common suction and/or discharge manifolds. For instance,a control for the two-compressor system may actuate both of thecompressors or either one of the two compressors. The two compressorsmay have different sizes to provide distinct stages of capacity duringpart-load operation. Rather than having a single level of capacity, arefrigerant system provided with tandem compressors would have severaldiscrete levels of capacity.

In the prior art, controls can be programmed to optionally actuate thetandem compressors. However, the capacity control provided by the tandemcompressors is increased or decreased in large discrete steps. It wouldbe desirable to provide the ability to improve system control capabilityto continuously vary capacity between these discrete steps to preciselymatch external load demands at a wide spectrum of environmentalconditions.

Variable speed drives are known for driving compressors at a variablespeed in a refrigerant system. By driving the compressor at a higher orlower speed, the amount of refrigerant that is compressed per unit oftime changes, and thus the system capacity can be adjusted.

Variable speed drives have not been utilized in refrigerant systemsincorporating tandem compressors, where a selected number of the tandemcompressors is driven by a variable speed drive, for the purpose ofvarying the system capacity to control temperature and humidity levelswithin the conditioned space.

SUMMARY OF THE INVENTION

In the disclosed embodiment of this invention, a variable speed drive isprovided into at least one compressor in a refrigerant system havingtandem compressors. By selectively controlling this one compressor,capacity adjustment between the discrete steps provided by tandemcompressor operation can be achieved.

A control identifies a desired cooling capacity, and then achieves thisdesired capacity by first actuating the tandem compressors to accuratelyapproximate the necessary capacity in the most efficient and reliablemanner. Then, the speed of the at least one compressor provided withvariable speed is changed incrementally. The capacity is then monitored.When a desired level is finally achieved, the at least one compressor isoperated at that new speed. If the capacity still needs to be adjusted,then the speed is again adjusted incrementally, and the resultingcondition is again monitored.

In disclosed embodiments, one of the tandem compressors may be providedwith the variable speed drive while the other is not. In otherembodiments, plural compressors are provided with a variable speeddrives.

Embodiments are disclosed which incorporate economizer cycles andunloader cycles into the schematic along with the variable speed drive.

These and other features of the present invention can be best understoodfrom the following specification and drawings, the following of which isa brief description.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 shows a first embodiment refrigerant system.

FIG. 1A shows other possible circuit schematics.

FIG. 1B shows other possible circuit schematics.

FIG. 1C shows other possible circuit schematics.

FIG. 2 shows a second embodiment refrigerant system.

FIG. 3 shows the capacity control provided by the prior art.

FIG. 4 shows the capacity control provided by the present invention.

FIG. 5 is a flowchart of a control algorithm according to the presentinvention.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT

A refrigerant system 20 is illustrated in FIG. 1. A compressor 22 isprovided with a variable speed drive 24. A second compressor 26 is notprovided with a variable speed drive, and operates in tandem with thecompressor 22. As shown, a shut-off valve 28 may allow the compressor 26to be isolated from the discharge manifold, should a control for thesystem determine that only the compressor 22 is necessary for achievinga given capacity. As is known, the compressors 22 and 26 deliverrefrigerant to a common discharge line 30 leading to a condenser 32.While the system 20 is illustrated as an air conditioning system, itshould be understood that the present invention would also apply to heatpumps and chillers.

As is known, the two compressors 22 and 26 may preferably be providedwith distinct capacities such that varying total levels of capacity canbe achieved by operating one or the other, or both of the compressors 22and 26. In this case, it is at the system designer's discretion toselect whether a larger or smaller compressor is provided with avariable speed drive. The decision will depend on many factors including(but not limited to) application requirements, cost, system operationefficiency, etc. An expansion device 34 is positioned downstream of thecondenser 32, and an evaporator 36 is located downstream of theexpansion device 34. A common suction line 38 leads to distinct suctionlines 39 for returning refrigerant to the compressors 22 and 26.

As also shown, an economizer circuit can be incorporated into the FIG. 1schematic. An economizer heat exchanger 40 receives a tapped refrigerantfrom a line 42 having passed through an economizer expansion device 44.As is known, by passing the tapped refrigerant through the expansiondevice 44, its pressure and temperature are lowered. Thus, in theeconomizer heat exchanger 40, this tapped refrigerant subcools arefrigerant in a main liquid line 45, which also passes through theeconomizer heat exchanger 40. The economizer function is known in theprior art, and allows increased capacity and/or efficiency of therefrigerant system 20.

As shown, the tapped refrigerant is returned through a line 46 to anintermediate compression point 48 in at least one of the compressors,here illustrated as compressor 22. While refrigerant in the tap line 42is shown flowing through the economizer heat exchanger 40 in the samedirection as refrigerant in the main liquid line 45, it should beunderstood that in a preferred embodiment, the two flows would actuallybe in counter-flow arrangement.

A bypass line 50 is also incorporated, and allows a portion ofrefrigerant from the intermediate compression point 48 in the compressor22 to be returned to the suction line 39. When it is desired to haveunloaded operation, a valve 52 is opened while the expansion device 44is preferably (but not necessarily) closed. In this way, refrigerantthat has been partially compressed by the compressor 22 will be returnedto the suction line 39, thus providing the unloading function.

It has to be understood that the economized compressor 22 may have morethan one injection port 48 and more than one associated economizer heatexchanger 40. Also, as known, the economizer heat exchanger arrangementcan be substituted by a flash tank. Further, multi-stage compressionsystem may be employed instead of a single economized compressor. Insuch multi-stage compressor system, one or several of the stages may beprovided with a variable speed drive.

As shown, electric motors 200 are associated with fans for blowing theair over the condenser 32 and evaporator 36. One or other of theseelectric motors 200 may be provided with a variable speed drive 202. Aworker of ordinary skill in the art would recognize when the variablespeed control of the fan, or other components such as a secondary looppump, motors associated with the refrigerant system might be desirable.

FIG. 1A shows another circuit schematic 100 wherein one of the twocompressors, e.g. compressor 22, is replaced by two compressor stages104 and 106. While both of the compressor stages 104 and 106 are shownconnected to the variable speed drive 102, only one stage or the othercould be connected instead. As shown, the return line 108 from theeconomizer heat exchanger extends simply between the two stages, ratherthan into compression chambers in either of the stages.

FIG. 1B shows another embodiment 110 wherein there are three compressorstages 112, 114 and 116. The variable speed drive 118 controls bothstages 114 and 116. Each of the stages is shown associated with anunloader valve 120. Two separate economizer heat exchangers 122selectively deliver refrigerant through lines 124 back to points betweenthe compressor stages. It is well known to a person ordinarily skilledin the art that a number of compression stages (as well as a number andparticular position of compression stages operating at variable speeds),a number of unloader valves and a number of economizer heat exchangersare at a designer freedom and depend on a particular application.

FIG. 1C shows another embodiment 130 wherein a first stage of thecompressor is provided by a pair of tandem compressors 134 and 136feeding a second compressor stage 138. As shown, an intermediatepressure refrigerant return line 140 extends between the stages. Avariable speed drive 132 is associated with the compressor 134 only. Ofcourse, many other schematics would come within the scope of thisinvention, including (but not limited to) a varying number of tandem andvariable speed compressors.

FIG. 2 shows a distinct embodiment 60, wherein the two tandemcompressors are replaced by a bank of four compressors. As shown,compressors 64 are each provided with a variable speed drive 62.Shut-off valves 66 are placed on the discharge lines for threecompressors 64, 68 and 70 to isolate those compressors when they arestopped by the system control. A common discharge manifold 72 leads to acondenser 74, an expansion device 76, and an evaporator 78. A controlfor this refrigerant system 60 is configured to operate the twocompressors 64 at variable speeds, and the two compressors 68 and 70 atfixed speed to achieve desired capacity.

A control for either refrigerant system 20 and 60 is able to identify adesired cooling capacity, and operate the tandem compressors and/or theeconomizer and unloader functions as necessary. Thus, as shown in FIG.3, a prior art system that incorporated the FIG. 1 schematic without thevariable speed drive could provide at least three stages A, B, and A+Bof capacity control. In fact, the schematic shown in FIG. 1 would haveeven more stages, in that the operation of the unloader valve andeconomizer function would provide additional capacity steps. However,for purposes of understanding the remainder of this invention, thesimplified schematic of FIG. 3 will suffice. As can be seen, there areseveral values between values A, B, and A+B that cannot be provided bythis prior art system. This is, of course, an oversimplification of thesystem, yet this does provide a good basis for understanding the presentinvention. The FIG. 2 embodiment would have many other levels ofcapacity control available as well.

FIGS. 3 and 4 are an oversimplification of the FIG. 1 embodiment and thecapacity levels it can provide. As mentioned, by operating the unloadervalve and economizer function, additional capacity steps can beachieved. However, a control for this system would operate one of thecompressors (e.g., compressor 26) that may be smaller than thecompressor 22 to provide the level A. The other compressor 22 can beoperated to provide the level B, with the compressor 26 stopped. Byoperating both compressors 22 and 26, the level A+B can be achieved.Within each of these levels, by increasing the speed of the motor forthe compressor 22, a ramp R above the step A, B, or A+B can be achieved.On the other hand, by slowing the speed, the opposite can occur to movea ramp downwardly from these values. A decision of switching between thecompressor speed adjustment and moving to a different mode of operationis usually based on the amount of required cooling, efficiency andreliability considerations. For instance, it may be unsafe to operatethe compressor below certain speed due to inadequate lubricationprovided to compressor elements. On the other hand, running compressorat a relatively high speed may be inefficient in comparison to switchingto an economizer mode of operation.

FIG. 5 shows how the ramps would typically be achieved with a standardvariable speed motor control as is known in the prior art. Ramps R asshown in FIG. 4 are an oversimplification. In fact, the controltypically moves in incremental steps, and then monitors the operation ofthe refrigerant cycle after that incremental change. Thus, there wouldbe a plurality of step changes along each ramp R, rather than theinfinite number of changes as is illustrated in FIG. 4. However, FIG. 4does provide a good illustration of the power of the present inventionto provide varying capacity.

It has to be noted that variable speed tandem compressors can beutilized in conjunction with other system components such as fans orpumps also operated at variable speeds.

Although preferred embodiments of this invention has been disclosed, aworker of ordinary skill in this art would recognize that certainmodifications would come within the scope of this invention. For thatreason, the following claims should be studied to determine the truescope and content of this invention.

1. A refrigerant system comprising: at least two tandem compressorsoperating in parallel, with at least one compressor having a variablespeed drive for varying a speed of said at least one compressor; acondenser downstream of said compressor and an evaporator downstream ofsaid condenser; and a control for selectively varying said speed of saidat least one compressor.
 2. The refrigerant system as set forth in claim1, wherein an economizer heat exchanger is positioned intermediate tosaid condenser and said evaporator, said economizer heat exchangerselectively receiving a tapped refrigerant to subcool a main refrigerantflow passing through said economizer heat exchanger, and said tappedrefrigerant being returned to least one of said compressors and saidcontrol being operable to vary the speed of at least said one compressorto provide variation in capacity control between a level with saideconomizer heat exchanger operational, and a level without saideconomizer heat exchanger operational.
 3. The refrigerant system as setforth in claim 2, wherein there are a plurality of intermediate portswhere said tapped refrigerant is returned to said at least one of saidcompressors.
 4. The refrigerant system as set forth in claim 1, whereinat least one of said two tandem compressors is provided by a multi-stagecompressor.
 5. The refrigerant system as set forth in claim 1, whereinsaid control changing said speed of said at least one compressor inincremental steps.
 6. The refrigerant system as set forth in claim 1,wherein at least one of said at least two compressors is provided withan unloader function.
 7. The refrigerant system as set forth in claim 1,wherein at least one of said at least two compressors is not providedwith a variable speed drive.
 8. The refrigerant system as set forth inclaim 1, wherein there are more than two of said at least twocompressors, and at least two of said compressors are provided with avariable speed drive.
 9. The refrigerant system as set forth in claim 1,wherein a fan or pump associated with a component other than thecompressor is also provided with the variable speed drive.
 10. Therefrigerant system as set forth in claim 1, wherein said at least twocompressors have different capacities.
 11. A method of controlling arefrigerant system comprising the steps of: (1) providing at least twotandem compressors operating in parallel, with at least one compressorhaving a variable speed drive for varying a speed of said at least onecompressor, providing a condenser downstream of said compressor and anevaporator downstream of said condenser, and a control for selectivelyvarying said speed of said at least one compressor to achieve varyinglevels of capacity control; and (2) determining a desired capacity, andoperating one or the other, or both of said at least two compressors,and varying a speed of said at least one compressor to achieve saiddetermined desired capacity.
 12. The method as set forth in claim 11,wherein an economizer function is provided with the refrigerant system,and selectively actuating said economizer function to provide additionalcapacity or increase operation efficiency if necessary to achieve thedesired capacity of step
 2. 13. The method as set forth in claim 12,wherein refrigerant from corresponding economizer heat exchangers isreturned to a plurality of ports associated with said at least twotandem compressors.
 14. The method as set forth in claim 11, wherein atleast one of said at least two compressors is provided by a multi-stagecompressor.
 15. The method as set forth in claim 11, wherein saidcontrol changes said speed of said at least one compressor inincremental steps.
 16. The method as set forth in claim 11, wherein anunloader function is provided to unload at least one of said at leasttwo compressors o achieve the desired capacity of step
 2. 17. The methodas set forth in claim 11, wherein at least one of said at least twocompressors is not provided with a variable speed drive.
 18. The methodas set forth in claim 11, wherein there are more than two of said atleast two compressors, and at least two of said compressors beingprovided with a variable speed drive, and said control varying the speedof said at least two variable speed driven compressors.
 19. The methodas set forth in claim 11, wherein said at least two compressors areprovided with different capacities.
 20. The method as set forth in claim11, wherein at least one fan or pump associated with another componentin said refrigerant system is provided with a variable speed drive.